Brain Protection and Meninges Quiz

Brain Protection

• The brain is protected by four main parts
• Bone (skull)
• Membranes (Meninges)
• Cerebrospinal Fluid
• Blood Brain Barrier

Meninges

• Three connective tissue membranes that lie externally to the CNS
• Functions (Fxns):
  • Cover/Protect CNS
  • Protect Blood Vessels/Enclose Venous Sinuses
  • Contain Cerebrospinal Fluid
  • Form Partitions within the skull
• Types of Meninges:
  • Dura Mater: Strongest outermost layer, a double layer sheet of fibrous tissue; encloses dural sinuses that collect venous blood from the brain and directs it into the Jugular Vein of the neck
  • Arachnoid Mater: Middle, web-like loose covering, filled with cerebrospinal fluid; contains the largest blood vessels serving the brain
  • Pia Mater: Innermost layer, delicate connective tissue and richly invested tiny blood vessels; only meninx that clings to the brain with every movement

Cerebrospinal Fluid (CSF)

• Found in and around the brain and spinal cord
• Forms a liquid cushion, providing buoyancy to CNS organs
• Functions (Fxns):
  • Reduces brain weight by 97% (prevents it from crushing itself)
  • Protects brain and spinal cord from blows or trauma
  • Helps nourish the brain/carry chemical signals
• Adults: Total CSF volume = 150 mL (about 1/2 a cup); Replaced every 8 hours = 500 mL of CSF formed every day
• Hydrocephalus: "Water on the Brain"
  • Babies: skull bones haven't fused yet
  • Adults: Tumor, Brain Damage, Trauma, etc.
  • Solution: Inserting a shunt into brain ventricles that drains fluid into the neck

Blood Brain Barrier (BBB)

• Maintains homeostasis (stable environment) for the brain
• Prevents the brain's exposure to blood-borne substances (e.g., various ions, waste, drugs, and other toxins) that may over-excite neurons
• Extremely selective, using tight junctions
• Ineffective against fats, fatty acids, O₂, and CO₂; Thus, nicotine, alcohol, and anesthetics affect the brain

Trauma

• Head injuries are the leading cause of accidental death in the US
• Brain damage is not only localized (e.g., blow to the head) but also caused by the ricocheting effect (e.g., shaking of the brain)
• Types of Trauma:
  • Concussions: Slight brain injury, mostly mild/short-lived; results in dizziness, seeing stars, brief loss of consciousness, but usually no permanent neurological damage
  • Contusions: Brain tissue destruction; eventually loses consciousness (coma) for many hours or perhaps a lifetime
• Following a head blow, death may result in subdural or subarachnoid hemorrhages
• Bleeding of ruptured blood vessels into these meningeal spaces
• Accumulating blood = ↑ intracranial pressure = Compression on brain tissue = Force of brainstem into foramen magnum = ⇓blood pressure, heart rate, respiration = death
• Solution: Surgery to remove the hematoma and repair the blood vessel
• Cerebral Edema: Swelling of the brain caused by increased water uptake
• Solution: Anti-inflammatory drugs (prednisone) & other steroids to help reduce injury aggravation

Cerebrovascular Accidents (CVAs)

• AKA strokes or brain attacks
• Most common nervous system disorder & 3^rd leading cause of death in the US
• Strokes: blood circulation to the brain is deprived/blocked (ischemia), and brain tissue dies
• Most common CVA = blockage of a cerebral artery by a blood clot
• Other causes include: Compression of brain tissue by hemorrhage or edema, or progressive narrowing of brain blood vessels
• 80% die from initial Cerebral Hemorrhage CVA attack; Survivors = partial paralysis (one side; may affect language, speech, etc.)
• Patients with CVAs caused by blood clots, usually have reoccurring clotting problems
• Solution: Anti-inflammatory drugs (prednisone) & other steroids to help reduce injury aggravation

Nervous System

• Key characteristics of the Nervous System:
  • Master controlling and communicating system of the body
  • Controls behavior, thought, actions, and emotions
  • Cells communicate by electrical signals, which are rapid, specific, and cause immediate responses
• Main functions of the Nervous System:
  • Sensory Input: Using sensory receptors to monitor changes in stimuli & gather sensory information
  • Integration: Processing sensory input & determining the course of action
  • Motor Output: Causing a response with effector organs (muscles and/or glands) based on the processed information
• Divisions of the Nervous System:
  • Central Nervous System (CNS):
    • Made up of the brain & spinal cord
    • Integrating & command center of the nervous system
    • Interprets incoming sensory info and dictates motor movement
  • Peripheral Nervous System (PNS):
    • Nervous System outside of CNS; nerves extending from the brain and spinal cord
    • Two Divisions of PNS:
      • Sensory (Afferent) Division: Brings information toward the CNS from sensory receptors (e.g., skin, skeletal muscles, joints). 
      • Motor (Efferent) Division: Transmits information away from the CNS to the effector organs & causing an effect
        • Somatic Nervous Sys: transmits signals from the CNS to skeletal muscles; voluntary movement
        • Autonomic Nervous Sys: transmits signals from the CNS to smooth muscle, cardiac muscle, and glands; automatic/involuntary movements

Neuron

• Made up of cell body, dendrites, and axon
• Cell Body (Soma): Contains the transparent spherical nucleus with nucleolus, contains usual organelles minus centrioles; most neuronal cell bodies are located within the CNS & are protected by bones & vertebrae
• Dendrites: Branch-like extensions from the cell body that receive electrical messages from other neurons, Many dendrites in the brain receive specialized information, messages are not nerve impulses but graded potentials (short distance signals sent between nerve cells).
• Axon: Long "tail" from cell body over which electrical signals are passed/transported; axons vary in length, with the longest traveling from 3–4 ft to control your big toe
• Myelin Sheath:
  • In PNS, made of Schwann Cells wrapped around axon, waxy/fatty coating, protects signal from being lost
  • If nervous tissue is myelinated, called white matter; if not myelinated, called gray matter
  • In CNS, Oligodendrocyte Cells provide insulation & protection for axons in the white matter; these are like Schwann Cells, but in the CNS, and they can attach to multiple axons at a time; there where Schwann cells can only attach to the axon of one neuron in the PNS.

Glial Cells

• Most abundant type of cell in nervous tissue; includes Astrocytes, Microglia, and Ependymal cells, Oligodendrocytes, Schwann Cells, and Satellite cells:
  • Astrocytes: most abundant, versatile, highly branched glial cells; twice as many astrocytes as there are neurons, cling to neurons & their synaptic endings, & cover capillaries making sure neurons get all the nutrition they need.
  • Microglia: small, oval-shaped cells with spiny processes; phagocytes that monitor the health of neurons
  • Ependymal cells: Line the cranial cavity and central canal of the spinal cord; produce and circulate cerebrospinal fluid.
  • Oligodendrocytes: found in the CNS, branched cells that wrap nerve fibers to insulate nerve impulses.
  • Schwann cells: found in the PNS, branched cells that wrap nerve fibers to insulate nerve impulses.

Myelin Sheath

• If myelinated, then electrical signals pass more rapidly down an axon
• In Multiple Sclerosis (MS), the body attacks and removes the myelin sheath; slows signal transmission
• Other nerves try to pick up the slack, but ultimately cannot replace all the damaged neurons

Action Potentials

• Changes in transmembrane potential that is spread over excitable membranes. Steps in the process of Action Potential
• Depolarization: Some stimulus hits the initial segment of a nerve, causes a shift allowing a small amount of Na+ into the cell; Transmembrane potential rises from -70mV to threshold value (usually -60 to -55mV); When a charge becomes more positive = Depolarization
• Repolarization: Once cell hits threshold, Na+ voltage-regulated channels open; Large rush of Na+ into the cell; Interior voltage rises to +30mV; Inactivation gates on Na+ channels close; K+ voltage-gated channels are opening and allowing K+ to move out of the cell; Driven out of the cell by the flood of + charges brought by the Na+ and the concentration gradient; Drops interior voltage back toward -70mV
• Hyperpolarization: Na+ channels remain inactivated until about -60mV; Na+ channels stay closed, but capable of reopening status; K+ channels start to close at about -70mV, but can't close fast enough so K+ moves out and drops interior of cell to -90mV (hyperpolarization); Gates return to normal position, cell goes to -70mV (this is the transmembrane/resting potential)

Synaptic Transmission

• Within the same neuron: Voltage Gated

• As Sodium floods into the first section of the nerve, it diffuses down the axon, carrying a positive charge

• As the positive charge flows down the axon, threshold value is reached

• At threshold, the sodium gates open and the action potential happens in this next section of the neuron

• In this way, the action potential is passed down (propagated) the entire length of the neuron

• From one neuron to the next:

  • When the action potential reaches the synaptic knobs, the vesicles fuse with the membrane and dump their neurotransmitter into the synapse
  • The neurotransmitter flows across the synapse to the next neuron's dendrites and triggers them to rise to threshold
  • Action potential flows down the “new” neuron

Test your knowledge on how the brain is protected by its structures, including the skull, meninges, and cerebrospinal fluid. This quiz covers the functions of the meninges and the different types that safeguard the central nervous system. Dive into the intricate details of brain anatomy and its protective mechanisms.